It is well known in the semiconductor art to use sintered aluminum nitride for insulating substrates because of its thermal conductivity. Many methods are known for preparing aluminum nitride powders which may be used for preparing sintered bodies having such properties. The two principal methods used for this purpose involve the direct nitriding of metallic aluminum powder at high temperature, in an atmosphere of nitrogen or ammonia gas, followed by milling the resulting nitride (see, for example, U.S. Pat. No. 3,108,887); and reducing alumina by firing alumina and carbon powder in nitrogen or ammonia gas, and thereafter pulverizing the resulting nitride (see, for example, U.K. Patent No. 1,100,865).
The first method is disadvantageous in that the resulting nitride can contain a large amount of impurities such as metallic aluminum or various metallic compounds. Generally, these impurities originate in the raw aluminum powder reactant, and/or from the pulverizing step.
In the second method there is always a large amount of oxygen present, generally from 2 to 5% by weight or more. The oxygen is present as unreacted alumina or other metallic oxides. It is known that the presence of these impurities drastically affects the properties of the sintered aluminum nitride body; in particular, the thermal conductivity decreases to such an extent that sintered aluminum nitride thus produced cannot be used as a substrate for most electronic components.
Various sintering cycle modifications have been proposed in the processing of aluminum nitrides, e.g., modifications in sintering temperatures, programmed increases from room temperatures to the sintering temperature (ramp temperature), and post-sintering cooling cycles. According to the prior art, notwithstanding changes in sintering cycles, the properties of the sintered bodies depend upon the properties of the initial AlN powder.
It is also known from Kuramoto et al. U.S. Pat. No. 4,618,592 that high purity aluminum nitride powders may be prepared by alumina reduction without using a pulverizing step, in an effort to obtain sintered bodies having high density and high thermal conductivity. According to this patent it is essential that aluminum nitride powders thus utilized possess an average particle diameter of not more than 2 microns, comprise at least 97% by weight aluminum nitride, and contain at most 1.5% by weight of bound oxygen; at most 0.5% by weight (calculated as metal) of metal or metal compound impurities, wherein at most 0.1% by weight of the metal is iron, chromium, nickel, cobalt, copper, zinc or titanium; or at most 0.5% by weight (calculated as carbon or silicon compounds) of carbon or silicon. According to the Kuramoto patent, it is not possible to obtain an aluminum nitride sintered body having good thermal conductivity and other properties without using such a high purity aluminum nitride powder.
More recently, it has been found that employing a particular sintering cycle, sintered aluminum nitride bodies having high thermal conductivities may be prepared, even without the use of aluminum nitride reactants having the relatively high purities required by Kuramoto. Such cycle involves:
(a) increasing the temperature of a compacted aluminum nitride body from room temperature to the sintering temperature (the ramp temperature) at a rate of no more than 250.degree. C. per hour; PA1 (b) sintering the body in an inert atmosphere at a temperature between about 1600.degree. and 1900.degree. C. and PA1 (c) thereafter cooling the sintered body at a rate of no more than 300.degree. C. per hour. PA1 (a) mixing an aluminum nitride powder with a sintering aid, the powder having a surface area of at least about 2.5 m.sup.2 /g, comprising at least about 95% by weight aluminum nitride, and incorporating impurities in no more than the following amounts: PA1 (b) compacting the mixture to form an aluminum nitride body; and PA1 (c) subjecting the body to the following sintering cycle:
The aforesaid sintering cycle is more fully described in copending Cox et al U.S. patent application Ser. No. 5,475 filed on Jan. 20, 1987 [KP. 3611; 2427-130-AP-00], owned by the assignee of the present invention; the disclosure of said application is incorporated herein by this reference.
It is among the objects of the present invention to provide an improved process for the production of sintered aluminum nitride bodies suitable for use as substrates for integrated circuits, or for other electrical components. The present process, like the process of the aforesaid Cox et al application, may utilize relatively impure aluminum nitride powders, and yet produce sintered bodies having high densities, thermal conductivities and other desired properties.